Re: [PATCH v4 6/7] blk-mq: fix freeze queue race

From: Jens Axboe
Date: Tue Sep 29 2015 - 11:01:43 EST

On 09/28/2015 08:48 AM, Tejun Heo wrote:

On Sun, Sep 27, 2015 at 10:06:05PM +0900, Akinobu Mita wrote:
void blk_mq_finish_init(struct request_queue *q)
+ mutex_lock(&q->mq_freeze_lock);
+ mutex_unlock(&q->mq_freeze_lock);

This looks weird to me. What can it race against at this point?

The possible scenario is described in commit log (1. ~ 7.). In summary,
blk_mq_finish_init() and blk_mq_freeze_queue_start() can be executed
at the same time, so this is required to serialize the execution of
percpu_ref_switch_to_percpu() by blk_mq_finish_init() and
percpu_ref_kill() by blk_mq_freeze_queue_start().

Ah, you're right. I was thinking that percpu_ref_switch_to_percpu()
being called after blk_mq_freeze_queue_start() would be buggy and thus
the above can't be enough but that is safe as long as the calls are
properly synchronized. Hmmm... maybe we should add synchronization to
those operations from percpu_ref side.

I think that would be very useful, it seems sort of half-assed if the caller side has to provide serialization for that.

Jens Axboe

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at